A bacterium's genome contains genes for proteins it needs only under specific conditions, and permanently producing all of these proteins regardless of whether they're currently needed would waste considerable cellular energy and resources. An operon solves this by grouping several related genes, ones whose protein products are typically needed together for a single task, under shared genetic control, so the whole group can be switched on or off together in direct response to a single environmental signal, rather than each gene needing its own separate, independent regulation.
Grouping related genes under one switch avoids managing each gene separately
Many cellular tasks require several different proteins working together, several enzymes needed in sequence to break down a particular nutrient, for instance. An operon places the genes coding for all these related proteins next to each other on the genome, under the control of a single shared regulatory region, so that a single regulatory event can switch the entire functional group on or off together as one coordinated unit. This is considerably more efficient than requiring the cell to separately regulate each individual gene in the group, since a single shared control point handles the whole set at once.
A regulatory signal directly reflects whether the operon's product is actually needed
An operon's shared regulatory region typically responds to a specific molecule, and whether that molecule's presence switches the operon on or off depends on the operon's particular biological purpose. In a classic example, the genes needed to break down a specific sugar stay switched off by default, since producing those digestive enzymes would be wasted effort when that sugar isn't actually present, but the presence of that same sugar directly triggers the operon to switch on, letting the cell produce exactly the enzymes it now actually needs, precisely when it needs them, rather than maintaining constant, wasteful production regardless of demand.
What we're still unsure about
The basic operon model, and its well-documented role in coordinating bacterial gene expression efficiently, are precisely characterised, extensively studied molecular biology, confirmed across decades of research on model bacterial systems. What's more genuinely an active area of ongoing research is understanding the full complexity of gene regulation in more complex organisms, since eukaryotic cells, including human cells, don't organise their genes into operons the same way bacteria do, and instead rely on a considerably more intricate, still incompletely mapped web of individual regulatory elements working together — researchers continue actively working to fully characterise these more complex eukaryotic regulatory networks, without yet having as complete and simple a picture as the bacterial operon model provides.
This sits inside Gene Regulation & Operons, one of eight topics in Genetics, one of six domains in Biology, one of seventeen subjects the app can quiz you on.